1. Field of the invention
The present invention relates to a process for distillation of styrenes by a heat pump system.
2. Description of the Prior Art
In distillation, heat is required for vaporizing the liquid and cooling is required for condensing the vaporized vapor. The amount of heat needed for vaporization is approximately the same as released in condensation. Particularly for purpose of improving the efficiency of distillation operation, it is important to decrease the amount of heat required for vaporization. For decreasing the amount of heat used in distillation columns, processes for distillation by a heat pump system are known (U.S. Pat. No. 4,056,444, Published Unexamined Japanese Patent Application No. 111,466/77, Published Examined Japanese Patent Applications No. 39,236/79). These processes for distillation comprise adiabatic compression of the vapor of low boiling point components discharged from the top of a distillation column with a compressor to elevate its temperature and use of latent heat of the vapor as a heat source for the reboiler. In these processes, the vapor from the top of the distillation column can be utilized as a heat source by externally adding a small quantity of energy. For this reason, these processes for distillation are said to be extremely useful from the viewpoint of thermal economy, as compared to ordinary distillation system.
Now, styrenes such as styrene, vinyltoluenes, .alpha.-methylstyrene, etc. are prepared by dehydrogenation of the corresponding ethylbenzenes such as ethylbenzene, ethyltoluenes, cumene, etc. These dehydrogenated oils (crude styrenes) are mainly composed of the resulting styrenes and the unreacted ethylbenzenes, which contain small quantities of light components such as benzene, toluene, etc. and heavy components such as styrene oligomers. Accordingly, it is necessary to separate and remove the light components and heavy components from the aforesaid dehydrogenated oils, upon preparation of styrenes. As such a process, a process for distillation using a plurality of distillation columns has been adopted.
However, the boiling points of styrenes are closely akin to those of ethylbenzenes. For example, in the case of styrene and ethylbenzene, the boiling point of the former is 145.2.degree. C. and the latter is 135.2.degree. C., under atmospheric pressure. The difference in boiling point therebetween is merely 9.0.degree. C. Therefore, in order to separate styrenes from ethylbenzenes by distillation, high efficiency of distillation is required and it is thus necessary to increase the number of stages of the distillation columns and the reflux ratio. For this reason, it is common that trays of 60 to 100 stages are set in a distillation column used for distillation of styrenes, in the case of a multi-stage column. It is also common that the reflux ratio is set to a considerably large value of approximately 10. Thus, the difference in pressure between the top and the bottom becomes large from 90 to 250 mmHg. In such a distillation column, therefore, a problem arises in that the energy to be supplied as a heat source becomes too large.
It is desired to adopt the process for distillation by the aforesaid heat pump system as a process for distillation of styrenes. However, when the difference in temperature between the top and the bottom in the distillation column becomes large, it is necessary to increase the compression ratio of the vapor of low boiling point components. Accordingly, it has not been considered to adopt the process for distillation using this heat pump system for distillation of styrenes containing components having close boiling points and also being capable of polymerization.